The invention relates to polishing semiconductor substrates and, more particularly, to polishing fluids for removing a barrier layer.
Circuit interconnects for semiconductor devices can be formed in a dielectric layer in which multiple trenches are arranged. The interconnects are formed by applying a barrier film over the underlying dielectric layer, followed by applying a metal layer over the barrier film. The metal layer is formed to a sufficient thickness to fill the trenches with metal. The interconnect fabrication process includes the use of a two-step chemical mechanical polishing (CMP) process.
CMP refers to a process of polishing a semiconductor wafer with a polishing pad and a polishing fluid. In a first polishing step, the metal layer is removed from the underlying barrier film and from the underlying dielectric layer. The metal layer is removed, both by abrasion applied by the polishing pad, and by chemical reaction with the polishing fluid accompanied by dissolution of the products of chemical reaction. The first polishing step removes the metal layer, leaving a smooth planar polished surface on the wafer, and further leaving metal in the trenches to provide circuit interconnects that are substantially planar with the polished surface. For example, Lee et al., in EP Pat. Pub. No. 1 072 662 A1, disclose the use of guanidine as an abrasion accelerator for accelerating an abrasive composition's dielectric removal rate in relation to a metal interconnect.
A typical first-step polishing process includes an aqueous solution having an oxidizing reagent, such as KNO3 or H2O2, in a polishing fluid to remove copper interconnects. The copper metal layer is removed by oxidation of the metal layer by the oxidizing reagent, and by abrasion originating from the polishing pad. Further, the polishing pad abrades the metal layer to minimize redeposition of the dissolved oxides from the solution onto the surface of the material being polished. The copper is removed from an underlying barrier film, for example, of tantalum (Ta) or tantalum nitride (TaN). The barrier film is more resistant to abrasion than is the copper, such that the barrier film acts as a polish stop for stopping the first-step polishing of copper. Further, oxidation of the surface of the barrier film by the polishing fluid will inhibit its removal during first-step polishing.
In a second polishing step, the barrier film is removed from the underlying dielectric layer. Second-step polishing can provide a smooth, planar polished surface on the dielectric layer. Ideally, the second polishing step does not remove excessive metal in the trenches. Excess metal removal in the second polishing step can contribute to dishing.
Dishing is a term of art that describes the formation of unwanted cavities in the circuit interconnects caused by removing excess metal in the trenches. Dishing can occur in both the first polishing step and in the second polishing step. The circuit interconnects are required to have precise dimensions that determine the electrical impedance of signal transmission lines, as provided by the circuit interconnects. Dishing in excess of acceptable levels causes dimensional defects in the circuit interconnects, which can contribute to attenuation of electrical signals transmitted by the circuit interconnects.
The second polishing step should cause minimal erosion. Erosion is a term of art that describes the unwanted lowering of the surface of the dielectric layer caused by removing some of the dielectric layer underlying the barrier film. Erosion that occurs adjacent to the metal in the trenches causes dimensional defects in the circuit interconnects, which can contribute to attenuation of electrical signals transmitted by the circuit interconnects. To minimize erosion, a polishing fluid for second-step polishing is desired to remove the barrier film with a higher removal rate than the removal rate for the dielectric layer.
The second polishing step should have a high removal selectivity for the barrier layer relative to the underlying layers. Removal selectivity is defined as a ratio of the removal rate of the barrier film, relative to the removal rate of the comparison layer, for example a dielectric layer or a metal film. For purposes of this specification, selectivity refers to the ratio in removal rate in distance per unit time, such as angstroms per minute. Thus, removal selectivity is a measure of the removal of the barrier film relative to the dielectric layer or the metal film. A high removal selectivity is desired for several integration schemes. Polishing with a polishing fluid that exhibits a high removal selectivity relative to the dielectric layer increases removal of the barrier layer or film and decreases removal of the dielectric layer. There is a desire for a polishing solution with an increased barrier layer removal rate and a decreased dielectric removal rate.